diff --git a/src/Builder.php b/src/Builder.php
index 5193619..7440186 100644
--- a/src/Builder.php
+++ b/src/Builder.php
@@ -26,6 +26,8 @@ class Builder
 
     protected $bins;
 
+    protected $dependencies = [];
+
     private static $default_options = [
         'install' => false,
     ];
@@ -55,7 +57,19 @@ class Builder
       
         $json = file_get_contents($this->path."/composer.json");
         
-        $composer = json_decode($json);      
+        $composer = json_decode($json);   
+        
+        if (isset($composer->require)) {
+            foreach ($composer->require as $package=>$constraint) {
+                if ($package == "php") {
+                    $this->dependencies[$package] = $constraint;
+                    print_info(" ! require php: %s", $constraint);
+                } elseif (strpos($package, "ext-") === 0) {
+                    $this->dependencies[$package] = $constraint;
+                    print_info(" ! require ext: %s %s", $package, $constraint);
+                }
+            }
+        }
 
         if (!isset($composer->bin) || !is_array($composer->bin)) {
             print_warn("No executable defined, building a library phar...");
@@ -159,10 +173,22 @@ class Builder
 
         $bins = (array)$bins;
 
+        $indexFile = "<?php\n";
+
+        $depcheck = null;
+        foreach ($this->dependencies as $dep=>$constraint) {
+            if ($dep == "php") {
+
+            } elseif (strpos($dep, "ext-") === 0) {
+                $ext = substr($dep, 4);
+                $depcheck.= 'if (!extension_loaded("' . $ext . '")) { @fwrite(STDERR, "Fatal: Missing required extension ' . $ext . '.\n"); exit(2); }' . PHP_EOL;
+            }
+        }
+
         if (!$index && count($bins)==0) {
-            $indexFile = '<?php require_once __DIR__."/vendor/autoload.php";';
+            $indexFile.= 'require_once __DIR__."/vendor/autoload.php";';
         } elseif (count($bins)>1) {
-            $indexFile = '<?php $bin=basename($argv[0],".phar"); switch($bin) {';
+            $indexFile.= '$bin=basename($argv[0],".phar"); switch($bin) {';
             for ($n = 1; $n<count($bins); $n++) {
                 $indexFile.= 'case '.var_export(basename($bins[$n]),true).': require_once __DIR__."/".'.var_export($bins[$n],true).'; break;';
             }
@@ -183,6 +209,8 @@ class Builder
             $index = "bootstrap.php";
         }
 
+        $indexFile = str_replace("<?php", "<?php ".$depcheck, $indexFile);
+
         $stub = "#!/usr/bin/env php\n".$phar->createDefaultStub($index);
         $phar->setStub($stub);